Method for fabricating a light emitting diode (LED) die having strap layer

ABSTRACT

A method for fabricating a light emitting diode (LED) die includes the steps of forming an epitaxial stack on a substrate having an n-type semiconductor layer, multiple quantum well (MQW) layers, and a p-type semiconductor layer. The method also includes the steps of forming a plurality of trenches in the n-type semiconductor layer, and forming a strap layer having conductive straps and contact areas in the trenches, forming an electrical insulator layer on the strap layer, forming an n-pad on the n-type semiconductor layer, and forming a p-pad on the p-type semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of Ser. No. 14/341,815 filed Jul. 27,2014 U.S. Pat. No. 9,419,195 B2.

FIELD

This application relates generally to light emitting diode (LED) diceand particularly to a light emitting diode (LED) die having a straplayer and to a method for fabricating the light emitting diode (LED)die.

BACKGROUND

A prior art light emitting diode (LED) die 10 in the form of a verticallight emitting diode (VLED) die is shown in FIGS. 1A-1C. As shown inFIG. 1A, the light emitting diode (LED) die 10 includes a substrate 12,and an epitaxial stack 14 on the substrate 12. The epitaxial stack 14includes an n-type semiconductor layer 16, a multiple quantum well (MQW)layer 18 in electrical contact with the n-type semiconductor layer 16configured to emit electromagnetic radiation, and a p-type semiconductorlayer 20 in electrical contact with the multiple quantum well (MQW)layer 18; the n-type semiconductor layer 16 could have multilayers ofvarious n-type doping including lightly doped layer (sometime is calledundoped layer) or buffered layer and may also have super latticelayer/layers; the p-type semiconductor layer 16 could have multilayersof various p-type doping level and electron blocking layer (EBL); it iswell known that the MQW layer 18 is comprised of many pairs of well andbarrier layers. The light emitting diode (LED) die 10 also includes amirror layer 22, a p-contact layer 24 in electrical contact with thep-type semiconductor layer 20, and a p-pad 28 in electrical contact withthe p-contact layer 24.

The light emitting diode (LED) die 10 also includes an n-pad 30comprised of multiple conductive n-trenches in electrical contact withn-conduct areas 27 on the n-type semiconductor layer 16. The lightemitting diode (LED) die 10 also includes an electrical insulator layer26 configured to electrically isolate the p-pad 28 and the n-pad 30. Asshown in FIG. 2, during a packaging process, the light emitting diode(LED) die 10 can be flip chip mounted to a module substrate 32 with thep-pad 28 bonded to a p-electrode 34 on the module substrate 32, and withthe n-pad 30 bonded to an n-electrode 36 on the module substrate 32.

One characteristic of the light emitting diode (LED) die 10 is that thep-pad 28 and the n-pad 30 are separated by a gap W_(GAP). The size ofthe gap W_(GAP) affects the output radiation of the light emitting diode(LED) die 10, particularly along the outside edge of the p-pad 28. Forexample, if the gap W_(GAP) is relatively large, then the distancebetween the p-pad 28 and the n-pad 30 would also be large, and theoutput radiation along the outside edge of the p-pad 28 would be low.The width W_(P-PAD) of the p-pad 28 is also dependent on the size of thegap W_(GAP), such that a smaller p-pad width W_(P-PAD) also produces alower output radiation. The size of the gap W_(GAP), along with thep-pad width W_(P-PAD) and the n-pad width W_(N-PAD), are set by thefabrication process, such that additional masks and additional processsteps, are required to vary the dimensions of these features.

It would be desirable for the gap W_(GAP) to be adjustable to permitoptimization of the radiation output of the light emitting diode (LED)die 10, particularly along the outside edge of the p-pad 28. Inaddition, an adjustable size for the gap W_(GAP) would allow the p-padwidth W_(P-PAD), and the n-pad width W_(N-PAD) to be optimized. Further,an adjustable gap W_(GAP) would facilitate the packaging process bypermitting flexibility in the alignment of the p-pad 28 and the n-pad 30to the electrodes 34, 36 on the module substrate 32. The presentdisclosure is directed to a light emitting diode (LED) die havingn-straps that permit the size of the gap W_(GAP), as well as the p-padwidth of the width W_(P-PAD) and the n-pad width W_(N-PAD) to beadjusted to provide optimal radiation output, and a large process windowfor packaging.

However, the foregoing examples of the related art and limitationsrelated therewith are intended to be illustrative and not exclusive.Other limitations of the related art will become apparent to those ofskill in the art upon a reading of the specification and a study of thedrawings. Similarly, the following embodiments and aspects thereof aredescribed and illustrated in conjunction with a light emitting diode(LED) die which are meant to be exemplary and illustrative, not limitingin scope.

SUMMARY

A light emitting diode (LED) die includes a substrate and an epitaxialstack on the substrate. The epitaxial stack includes a first-typesemiconductor layer, a multiple quantum well (MQW) layer in electricalcontact with the first-type semiconductor layer configured to emitelectromagnetic radiation, and a second-type semiconductor layer inelectrical contact with the multiple quantum well (MQW) layer. The lightemitting diode (LED) die also includes a first pad in electrical contactwith the first-type semiconductor layer, and a second pad in electricalcontact with the second-type semiconductor layer. The light emittingdiode (LED) die also includes a strap layer having conductive straps andcontact areas formed in trenches in the second-type semiconductor layer.The strap layer spreads current over the contact areas, and permits agap width between the first pad and the second pad, as well as thewidths of the first pad and the second pad to be adjusted.

A method for fabricating the light emitting diode (LED) die includes thesteps of forming an epitaxial stack on a substrate having a first-typesemiconductor layer, multiple quantum well (MQW) layers, and asecond-type semiconductor layer. The method also includes the steps offorming a plurality of trenches in the second-type semiconductor layer,and forming a strap layer having conductive straps and contact areas inthe trenches, forming an electrical insulator layer on the strap layer,forming a first pad on the first-type semiconductor layer, and formingsecond pad on the second-type semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in the referenced figures of thedrawings. It is intended that the embodiments and the figures disclosedherein are to be considered illustrative rather than limiting.

FIG. 1A is a schematic cross sectional view of a prior art lightemitting diode (LED) die taken along section line 1A-1A of FIGS. 1B and1C;

FIG. 1B is a schematic cross sectional view of the prior art lightemitting diode (LED) die taken along section line 1B-1B of FIG. 1A;

FIG. 1C is a schematic plan view of the prior art light emitting diode(LED) die;

FIG. 2 is a schematic side elevation view of the prior art lightemitting diode (LED) die flip chip mounted to a module substrate;

FIG. 3A is a schematic bottom view of a flip chip light emitting diode(LED) die having an n-strap layer;

FIG. 3B-1 is a schematic transverse sectional view along an innersection that is planar to a surface of an n-conduct area of the flipchip light emitting diode (LED) die which has rectangular strap design;

FIGS. 3B-2 and 3B-3 are a schematic transverse sectional view along aninner section that is planar to a surface of an n-conduct area of theflip chip light emitting diode (LED) die which has non rectangular strapdesign in these examples they are elongated and trapezoid to furtherenhance current spreading.

FIG. 3C is a schematic cross sectional view taken along section line A1y and A2 y in FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating a y-directioncross section of the flip chip light emitting diode (LED) die;

FIG. 3D is a schematic cross sectional view taken along section line A2x in FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating an x-direction crosssection of the flip chip light emitting diode (LED) die;

FIG. 3E is a schematic cross sectional view taken along section line A1x in FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating an x-direction crosssection of the flip chip light emitting diode (LED) die;

FIG. 3F is a schematic cross sectional view taken along section line Byin FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating a y-direction crosssection of the flip chip light emitting diode (LED) die;

FIG. 3G is a schematic cross sectional view taken along section line Bxin FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating an x-direction crosssection of the flip chip light emitting diode (LED) die;

FIG. 3H is a schematic cross sectional view taken along section line Cyin FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating a y-direction crosssection of the flip chip light emitting diode (LED) die;

FIG. 3I is a schematic cross sectional view taken along section line Cxin FIGS. 3A and 3B-1, 3B-2, 3B-3 illustrating an x-direction crosssection of the flip chip light emitting diode (LED) die;

FIGS. 4A-4D are schematic cross sectional views illustrating steps in amethod for fabricating the flip chip light emitting diode (LED) die ofFIGS. 3A-3I; and

FIG. 5 is a schematic side elevation view of the flip chip lightemitting diode (LED) die mounted to a module substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood that when an element is stated as being “on”another element, it can be directly on the other element or interveningelements can also be present. However, the term “directly” means thereare no intervening elements. In addition, although the terms “first”,“second” and “third” are used to describe various elements, theseelements should not be limited by the term. Also, unless otherwisedefined, all terms are intended to have the same meaning as commonlyunderstood by one of ordinary skill in the art.

Referring to FIGS. 3A-3H, a light emitting diode (LED) die 40 isillustrated. Although a square light emitting diode (LED) is described,it is to be understood that the concepts disclosed herein can be adaptedto other types of (LED) dice or shapes such as rectangular or hexagonal(LED) dice. In addition, it is also be understood that the conceptsdisclosed herein can be adapted to other types of semiconductor deviceswhere one needs to use a metal strap layer having better electricalconductivity than the semiconductor layer to deliver electron from oneside of one polarity semiconductor layer to the farther side of the samepolarity semiconductor layer before moving vertically through theopposite polarity semiconductor layer. In addition, for simplicity,various elements of the light emitting diode (LED) die 40 are notillustrated in all of the figures. As shown in the cross sections (FIGS.3C-3H), the light emitting diode (LED) die 40 includes a substrate 42and an epitaxial stack on the substrate 42 comprised of an n-typesemiconductor layer 46, multiple quantum well (MQW) layer 48 inelectrical contact with the n-type semiconductor layer 46 configured toemit electromagnetic radiation, and a p-type semiconductor layer 50 inelectrical contact with the multiple quantum well (MQW) layer 48.

As shown in FIG. 3A, the light emitting diode (LED) die 40 also includesan n-pad 52 in electrical contact with the n-type semiconductor layer46, which forms a cathode (−), and a p-pad 54 in electrical contact withthe p-type semiconductor layer 50, which forms an anode. The n-pad 52and the p-pad 54 can comprise a conductive material, could be single ormultiple layers comprising of metal or metal alloy containing W, Ti, Mo,Al, Cu, Ni, Ag, Cr, Pt, Sn, Au or Co, Cu—Co, Cu—Mo, TiW, AuSn or SnCu,or Ni/Cu or Ni/Cu—Mo.

The n-type semiconductor layer 46 can comprise n-GaN. Other suitablematerials for the n-type semiconductor layer 46 include n-AlGaN,n-InGaN, n-AlInGaN, AlInN and n-AlN. The multiple quantum well (MQW)layer 48 includes one or more quantum wells comprising one or morelayers of InGaN/GaN, AlGaInN, AlGaN, AlInN and AlN. The multiple quantumwell (MQW) layer 48 can be configured to emit electromagnetic radiationfrom the visible spectral region (e.g., 400-770 nm), the violet-indigospectral region (e.g., 400-450 nm), the blue spectral region (e.g.,450-490 nm), the green spectral region (e.g., 490-560 nm), the yellowspectral region (e.g., 560-590 nm), the orange spectral region (e.g.,590-635 nm) or the red spectral region (e.g., 635-700 nm). The p-typesemiconductor layer 50 can comprise p-GaN. Other suitable materials forthe p-type semiconductor layer 50 include p-AlGaN, p-InGaN, p-AlInGaN,p-AlInN and p-AlN.

As shown in the cross sections (FIGS. 3C-3H), the light emitting diode(LED) die 40 also includes a mirror layer 56 on the p-type semiconductorlayer 50 comprising a highly reflective metal layer configured toreflect the electromagnetic radiation emitted by the multiple quantumwell (MQW) layer 48 outward from the (LED) die 40. By way of example,the mirror layer 56 can comprise multiple layers, such as Ni/Ag/Ni/Au,Ag/Ni/Au, Ti/Ag/Ni/Au, Ag/Pt or Ag/Pd or Ag/Cr, formed by depositing analloy containing Ni, Ag, Au, Cr, Pt, Pd, or Al.

As shown in the cross sections (FIGS. 3C-3H), the light emitting diode(LED) die 40 also includes an n-strap layer 60 in electrical contactwith the n-pads 52 and with the n-type semiconductor layer 46, and anelectrical insulator layer 64 configured to electrically insulate then-strap layer 60. As shown in FIGS. 3B-1, 3B-2, 3B-3, the n-strap layer60 includes a plurality of n-contact areas 44 formed in n-trenches 68 inthe n-type semiconductor layer 46. The n-contact areas 44 spread outcurrent and permit low resistance electrical paths with the n-typesemiconductor layer 46.

The n-strap layer 60 can comprise a same metal as the n-pad 52 or adifferent metal than the n-pad 52, n-strap layer could be single ormulti layers. Suitable metals include W, Ti, Mo, Al, Cu, Ni, Ag, Cr, Pt,Au or Co, or metal alloy containing W, Ti, Mo, Al, Cu, Ni, Ag, Cr, Pt,Au or Co such as Cu—Co, TiW, or Cu—Mo. The electrical insulator layer 64can comprise an electrically insulating material, such as an oxide(e.g., SiO₂, TiO₂ Al₂O₃, HfO₂, Ta₂O₅), a polymer (e.g., epoxy, parylene,polyimide, photoresist, EPON Resin Su-8), a nitride (e.g. SiliconNitride Si₃N₄ or Silicon Oxinitride SiNxOy, or a glass (e.g., SiO2, PSG,BPSG borophosphosilicate glass).

As shown in the cross sections (FIGS. 3C-3H), the light emitting diode(LED) die 40 also includes a p-contact layer 62 on the mirror layer 56in electrical contact with the p-type semiconductor layer 50 and withthe p-pad 54. The insulator layer 64 electrically insulates thep-contact layer 62 from the n-pads 52. The p-contact layer 62 can beformed of the same materials as previously described for the n-straplayer 60.

As shown in the cross sections (FIGS. 3C-3H, 4A), the n-trenches 68 areformed in the n-type semiconductor layer 46 with a selected depth Dt, aselected width W_(T) and a selected geometrical shape; the n-trenches 68are etched into the n-type semiconductor layer 46 to an optimum depth Dtto maximize the current conduction to achieve the lowest n-contactresistance resulting in low forward voltage and higher photon radiationoutput at operating current; this optimized Dt is achieved byexperimentation where one would vary the trench depth into the n-typesemiconductor layer 46 to achieve the desired value of n-contactresistance and/or current spreading to achieve optimum photon radiationoutput at desired operating current.

The n-strap layer 60 includes the n-contact areas 44 (FIGS. 3B-1, 3B-2,3B-3), which are formed along the bottom surface of the n-trenches 68 inelectrical contact with the surface of the n-type semiconductor layer46. The n-strap layer 60 forms a plurality of conductive straps 66having a width that smaller than the width of the n-trenches 68. Inaddition, as shown in FIGS. 3B-2 and 3B-3, the n-strap layer 60 can forma plurality of conductive straps 66 with non-rectangular shapes such astrapezoid to further enhanced the current spreading. In addition, asshown in FIG. 3A, the conductive straps 66 can be formed with a lengththat is selected to span the width of the gap W_(G) between the n-pad 52and the p-pad 54. Further, the geometrical shape of the n-trenches 68determines the geometrical shape of the n-contact areas 44 (FIG. 3B-1).As shown in FIG. 3B-1, at least some of the n-trenches 68 can compriseelongated, could be rectangular shaped trenches or non-rectangularshaped trenches, which are oriented such that a portion of n-trenches 68would go under the p-pad 54 where the p-pad 54 is above a portion of then-strap 60 but would be isolated from the conductive straps 66; at leastsome of the elongated n-trenches 68 are oriented such that a portion ofn-trenches 68 would go under the n-pad 52 where the n-pad 52 is above aportion of the n-strap 60 but would be making contact to a portion ofthe conductive straps 66 underneath the n-pad. As also shown in FIG.3B-1, at least some of the n-trenches 68 can comprise more than oneshape such as rectangular with circular members, rectangular withcircular and ellipse members, short and long rectangular members,rectangular and square members, elongated and non-elongated members orcombination of different shapes. Again the size, number and shape of then-trenches 68 are intended to be exemplary and not limiting in scopeAgain the geometrical shape of the elongated n-trenches 68 also can benon rectangular shaped trenches, like small n-end with big p-end or bign-end with small p-end (FIGS. 3B-2 and FIG. 3B-3).

The n-strap layer 60 performs several functions in the light emittingdiode (LED) die 40. A first function of the n-strap layer 60 is toincrease the n-contact areas 44 (FIGS. 3B-1, 3B-2, 3B-3) with the n-typesemiconductor layer 46. This improves current flow in the light emittingdiode (LED) die 40 and increases the radiation output. The n-strap layer60 also forms the conductive straps 66 having n-ends that electricallycontacts the n-pad 52, and having p-ends that extend under the p-pad 54but isolated from the p-pad 54. The n-strap layer 60 enhances thecurrent spreading by delivering the electron to the outer edge of then-type semiconductor layer under the p-pad 54 due to better electricalconductivity of the metal strap layer 66 as comparing to the n-typesemiconductor layer 46, resulting in higher the radiation output alongthe outer edge of the p-pad 54 due to the resistance path that theelectron would travel between the outer edge of the p-pad 54 and then-strap layer 60 can be less than it would be without the n-strap layer60. The n-strap layer 60 also allows the width W_(P) (FIG. 3A) of thep-pad 54, the width W_(N) (FIG. 3A) of the n-pad 52, and the gap widthW_(G) (FIG. 3A) between the n-pad 52 and the p-pad 54 to be adjustable,without requiring different masks during the fabrication process. Inaddition, this width adjustability increases the process window during asubsequent packaging process for the light emitting diode (LED) die 40,as the size and spacing of the n-pad 52 and the p-pad 54 can be matchedto the size and spacing of an n-electrode 74 (FIG. 5) and a p-electrode76 (FIG. 5) on a module substrate 78 (FIG. 5).

Referring to FIGS. 4A-4D, steps in a method for fabricating the lightemitting diode (LED) die 40 are illustrated. Initially, as shown in FIG.4A, the substrate 42 can be provided. For a wafer level fabricationprocess, the substrate 42 can be in the form of a wafer that can laterbe separated into multiple light emitting diode (LED) dice 40. Thesubstrate 42 can comprise a suitable material, such as sapphire, siliconcarbide (SiC), silicon, germanium, zinc oxide (ZnO), or gallium arsenide(GaAs).

As also shown in FIG. 4A, an epitaxial stack comprised of the n-typesemiconductor layer 46, the multiple quantum well (MQW) layer 48 and thep-type semiconductor layer 50 can be formed on the substrate 42 using asuitable deposition process such as vapor phase epitaxy (VPE), molecularbeam epitaxy (MBE) or liquid phase epitaxy (LPE). In the illustrativeembodiment, the n-type semiconductor layer 46 comprises n-GaN and thep-type semiconductor layer 50 comprises p-GaN. Rather than GaN, then-type semiconductor layer 46, and the p-type semiconductor layer 50 cancomprise various other compound semiconductor materials, such as AlGaN,InGaN, and AlInGaN.

As also shown in FIG. 4A, the mirror layer 56 can be formed on thep-type semiconductor layer 50 and a thin protective layer (not shown),such as a layer of SiO₂ can be formed on the mirror layer 56. By way ofexample, the mirror layer 56 can comprise multiple layers, such asNi/Ag/Ni/Au, Ag/Ni/Au, Ti/Ag/Ni/Au, Ag/Pt or Ag/Pd or Ag/Cr, formed bydepositing an alloy containing Ag, Au, Cr, Pt, Pd, or Al. A thickness ofthe mirror layer 56 can be less than about 1.0 μm. High temperatureannealing or alloying of the mirror layer 56 can be used to improve thecontact resistance and adhesion of the mirror layer 56 to the p-typesemiconductor layer 50. For example, the annealing or alloying processcan be conducted at a temperature of at least 150° C. in an inertenvironment (e.g., an atmosphere containing little or no oxygen,hydrogen, or neither oxygen nor hydrogen).

As also shown in FIG. 4A, a suitable process can be used to form then-trenches 68 in the epitaxial stack that can extend a selected distanceinto the n-type semiconductor layer 46. In addition, the n-trenches cancomprise elongated rectangular elements corresponding to the rectangularn-contact areas 44 in FIGS. 3B-1, 3B-2, 3B-3 or circular elementscorresponding to the circular n-contact areas in FIGS. 3B-1, 3B-2, 3B-3.For elongated rectangular trenches, a width W_(T) of the n-trenches 68can be in a range of from about 0.1 μm to about 300 μm. For circulartrenches, a diameter D of the n-trenches can also be in a range of fromabout 0.1 μm to about 300 μm. One suitable process for forming then-trenches 68 comprises dry etching through a hard mask. After thetrench forming process, a liquid or a solvent can be used to remove theetch mask, or other protective coating. As another option, p-trenches(not shown) could also be formed in the p-type semiconductor layer 50.

Next, as shown in FIG. 4B, the n-strap layer 60 can be formed in then-trenches 68 on the surface of the n-type semiconductor layer 46. Asalso shown in FIG. 4C, the p-contact layer 62 can be formed on thep-type semiconductor layer 50 using the same deposition process as forthe n-strap layer 60, or using a separate deposition process. Then-strap layer 60 and the p-contact layer 62 can each comprise a metallayer, a metal alloy or a metal stack as previously described. Then-strap layer 60 and the p-contact layer 62 can each be formed using asuitable deposition process, such as an electro-deposition process or anelectroless deposition process, to a desired thickness (e.g., 1 μm to500 μm) and with the previously described sizes and shapes. Othersuitable deposition processes for forming the n-strap layer 60 and thep-contact layer 62 can include chemical vapor deposition (CVD),metal-organic chemical vapor deposition (MOCVD), plasma enhancedchemical vapor deposition (PECVD), molecular beam epitaxy (MBE), vaporphase epitaxy (VPE), physical vapor deposition (PVD), evaporation, andplasma spray.

Next, as also shown in FIG. 4C, the electrical insulator layer 64 can beformed using a suitable deposition or growth process. As previouslydescribed, the electrical insulator layer 64 can comprise anelectrically insulating material, such as an oxide (e.g., SiO₂, TiO₂Al₂O₃, HfO₂, Ta₂O₅), a polymer (e.g., epoxy, parylene, polyimide,photoresist, EPON Resin Su-8), a nitride (e.g. Si₃N₄), or a glass (e.g.,BPSG borophosphosilicate glass).

Next, as shown in FIG. 4D, the p-pad 54 and the n-pad 52 can be formedon the electrical insulator layer 64. The p-pad 54 and the n-pad 52 cancomprise a metal layer, a metal alloy or a metal stack formed using asuitable process such as etching an initially deposited layer (e.g.,subtractive process), or alternately patterned deposition through a mask(e.g., additive process). For example, a deposition process, such as anelectro-deposition process or an electroless deposition process can beused to deposit metal layer to a desired thickness (e.g., 1 μm to 500μm). Other suitable deposition processes include chemical vapordeposition (CVD), metal-organic chemical vapor deposition (MOCVD),plasma enhanced chemical vapor deposition (PECVD), molecular beamepitaxy (MBE), vapor phase epitaxy (VPE), physical vapor deposition(PVD), evaporation, and plasma spray.

Following these process steps, a separating process can be performedusing a suitable process such as laser dicing, sawing, breaking, airknifing or water jetting to singulate the individual light emittingdiode (LED) die 40. For simplicity, these steps are not shown. By way ofexample, the light emitting diode (LED) die 40 can have a squareperipheral shape having a size range of at least 100 μm on a side (S).However, this shape and size range is by way of example and otherpolygonal, circular and oval shapes and size ranges can be employed. Arepresentative size range for the width W_(P) of the p-pad 54 can befrom at least 20 μm. A representative size range for the gap width W_(G)can be from at least 20 μm. A representative size range for the widthW_(T) of the n-trenches 68 can be from at least 10 μm and the length ofthe n-trenches 68 can be from at least 10 μm. Again, all of these sizeranges are intended to be exemplary and not limiting in scope.

The fabrication method with the n-strap layer 60 permits the size of thegap W_(G), as well as the width W_(P) of the p-pad 54 and the widthW_(N) of the n-pad 52 to be adjusted to provide optimal radiation outputand a large process window for packaging. In addition, the lightemitting diode (LED) die 40 and fabrication method provide at leastseveral advantages. First, the electrical flow distance between thep-pad 54 and n-pad 52 shrinks due to the n-strap layer 60. This improvesthe output radiation, while decreasing or maintaining fabrication costs,regardless of the values for the gap width W_(G), the width W_(P) of thep-pad 54, and the width W_(N) of the n-pad 52. Additionally, thefabrication method accommodates different sizes for the gap width W_(G),the width W_(P) of the p-pad 54, and the width W_(N) of the n-pad 52.This provides design flexibility without increasing manufacturing costs.Second, the n-strap layer 60 provides additional flexibility fordesigning the sizes of the p-pad 54, the n-pad 52, and the gap widthW_(G), which facilitates the packaging process. Third, additional masksand additional process steps are not required, which not only improvesyield, but also decreases the cost of manufacturing, and saves the timeof manufacturing as well.

As shown in FIG. 5, the light emitting diode (LED) die 40 can be flipchip mounted to a module substrate 78 to form a package system 82.During a flip chip bonding process, the p-pad 54 can be bonded to thep-electrode 76 on the module substrate 78 to provide an anode, and then-pad 52 can be bonded to the n-electrode 74 on the module substrate 78to provide a cathode. Suitable bonding processes include soldering,eutectic, reflow or conductive adhesive bonding.

Thus the disclosure describes an improved flip chip light emitting diode(LED) die and a method for fabricating the (LED) die. While thedescription has been with reference to certain preferred embodiments, aswill be apparent to those skilled in the art, certain changes andmodifications can be made without departing from the scope of thefollowing claims.

What is claimed is:
 1. A method for fabricating a light emitting diode(LED) die comprising: forming an epitaxial stack on a substrate having afirst-type semiconductor layer comprising a p-type semiconductor layer,a multiple quantum well (MQW) layer, and a second-type semiconductorlayer comprising an n-type semiconductor layer; forming a plurality oftrenches in the second-type semiconductor layer; forming an n-straplayer on the epitaxial stack comprising a plurality of conductive strapsand contact areas in the trenches, with the contact areas in electricalcontact with the second-type semiconductor layer and with the contactareas configured to provide a total contact area; forming an electricalinsulator layer on the n-strap layer; forming a first pad on thefirst-type semiconductor layer; forming a second pad on the second-typesemiconductor layer, with the first pad and the second pad separated bya gap; and adjusting a size of the gap by selection of a length of theconductive straps.
 2. The method of claim 1 wherein the trenchescomprise elongated rectangular trenches or circular trenches formed inthe second-type semiconductor layer to a selected depth.
 3. A method forfabricating a light emitting diode (LED) die comprising: forming anepitaxial stack comprising a p-type semiconductor layer, a multiplequantum well (MQW) layer in electrical contact with the p-typesemiconductor layer configured to emit electromagnetic radiation, and ann-type semiconductor layer in electrical contact with the multiplequantum well (MQW) layer having a trench; forming a p-pad in electricalcommunication with the p-type semiconductor layer; forming an n-pad inelectrical contact with the n-type semiconductor layer, with the p-padand the n-pad separated by a gap; forming an n-strap layer comprising aconductive material in the trench in electrical contact with the n-padand with the n-type semiconductor layer, the strap layer configured toconduct current from the n-pad through the trench and across the n-typesemiconductor layer in an area under the p-pad, with the area having aselected size and shape; forming an electrical insulator layer on thestrap layer configured to electrically insulate the conductive materialfrom the p-pad; and adjusting a size of the gap by selection of a lengthof the conductive straps.
 4. The method of claim 3 wherein the trenchcomprises an elongated rectangular shaped trench formed in the n-typesemiconductor layer to a selected depth.
 5. The method of claim 3wherein the trench has a shape selected from the group consisting ofrectangular, circular, square, elliptical, trapezoidal and a combinationof these shapes.
 6. The method of claim 3 further comprising forming amirror layer on the p-type semiconductor layer configured to reflect theelectromagnetic radiation emitted by the multiple quantum well (MQW)layer outward from the (LED) die.
 7. A method for fabricating a lightemitting diode (LED) die comprising: providing a substrate; forming anepitaxial stack on the substrate comprising a p-type semiconductorlayer, a multiple quantum well (MQW) layer in electrical contact withthe p-type semiconductor layer configured to emit electromagneticradiation, and an n-type semiconductor layer in electrical contact withthe multiple quantum well (MQW) layer having a plurality of trenches;forming a p-pad in electrical contact with the p-type semiconductorlayer; forming an n-pad in electrical contact with the n-typesemiconductor layer, with the p-pad and the n-pad separated by a gap;forming an n-strap layer in electrical contact with the n-pad and withthe n-type semiconductor layer, the n-strap layer comprising a pluralityof conductive straps in the trenches having a plurality of n-ends andn-contact areas in the trenches configured to spread current across then-type semiconductor layer and a plurality of p-ends extending subjacentto the p-pad configured to electrically contact the p-pad; forming anelectrical insulator layer on the n-strap layer configured toelectrically insulate the p-ends of the conductive straps from thep-pad; and adjusting a size of the gap by selection of a length of theconductive straps.
 8. The method of claim 7 wherein the conductivestraps have a shape selected to spread the current.
 9. The method ofclaim 7 further comprising providing a module substrate having ap-electrode and an n-electrode, and bonding the p-pad to the p-electrodeand the n-pad to the n-electrode.
 10. The method of claim 7 wherein atleast some of the trenches comprise elongated rectangular shapedtrenches formed in the n-type semiconductor layer to a selected depth.11. The method of claim 7 wherein the trenches have shapes selected fromthe group consisting of rectangular, circular, square, elliptical,trapezoidal and a combination of these shapes.